In the growing front regions of mutant strain of Neurospora, designated band (bd), an oscillation was found in the percentage of fatty acids that were unsaturated. This oscillation had a period of 22 hrs, a minimum of 37% and a maximum of 83% of unsaturated fatty acids. The total amount of fatty acids (saturated plus unsaturated) was relatively constant during the experiment. The effects of exogenous fatty acids on the growth rate and on the conidiation rhythm were determined for the bd strain and for the bd, cel strain. This latter strain has a partial requirement for saturated fatty acids. Palmitic, stearic, oleic, linoleic, and lineolenic acids were added to the media either as individual free fatty acids or as water-soluble Tween detergents. The concentrations of fatty acids added to the media had little effects on the rapid growth of either strain. No significant deviation from the 23 hr conidiation rhythm was noted in the bd strain due to any of these supplements. On the other hand, the bd, cel strain had a shorter period (21 hrs) when grown in the presence of Tween containing saturated fatty acids, and a longer (26,33 hr) period when supplied with the Tween containing oleic or linolenic acid, respectively. The period-lengthening effect of the unsaturated fatty acids could be reversed by the addition of saturated fatty acids.